Zeolites are crystalline aluminosilicates of Group IA and IIA whose empirical formulae can be written as: EQU M.sub.2/n O.Al.sub.2 O.sub.3..times.SiO.sub.2.y H.sub.2 O
where n is the cation charge and x and y are the number of SiO.sub.2 and H.sub.2 O entities respectively. There are approximately 40 natural mineral Zeolites and over 150 synthetic types known. These are complex crystalline inorganic polymers based on an tetrahedral network in which Al.sup.3+ and Si.sup.4+ are connected by oxygen in such a way that each is bonded to four oxygen atoms. The substitution of aluminium for silicon produces a deficiency in electrical charge that must be locally neutralised by the presence of an additional positive ion (eg Na.sup.+) within the interstices of the structure. This framework structure contains channels or interconnected voids that are occupied by the cation and water molecules. The cations are mobile and can undergo ion exchange. The water may be removed reversibly, which leaves intact a crystalline host structure permeated by micropores which may account for up to 50% of the volume and which can trap molecules of a size comparable with the size of the micropores, and in this way Zeolites may be used to absorb small molecules such as water, ethanol etc.
The ease with which such molecules can enter and leave the micropores depends inter alia on the characteristics of the Zeolite. For example if the micropores are `one dimensional` ie in the form of non-intersecting tunnels through the lattice, movement of the molecules into and out of the Zeolite will be restricted. If the pore apertures approximate to the size of the molecules then again such movement of the molecules will be restricted.
One mechanism by which absorbed molecules may be lost from the Zeolite is that of their exchange with molecules in the environment. The rate or exchange may be expressed in terms of a hair-life (T1/2), the time in which hair of the absorbed molecules will exchange. It can be shown that T1/2 is proportional to the square of the radius (r) of the Zeolite crystallite, ie for a long T1/2 a large r is required.
The ability of Zeolites to absorb and retain molecules is exploited in various ways industrially, for example as `molecular sieves` to selectively absorb certain molecules on the basis of size, especially absorption of water in drying processes. One potentially important application of water-absorbing Zeolites is in the nuclear industry, in the disposal of waste water containing radioactive tritium oxide T.sub.2 O. T.sub.2 O cannot be allowed to enter and pollute the environment on any significant scale, and consequently waste water containing T.sub.2 O must be stored until the tritium has decayed to an extent that its radioactivity is no longer a problem.
As solids are more convenient to store than liquids, it is preferred to convert such waste water into solids. One known method of doing this, used at present, is to absorb this water into a Zeolite. The Zeolite most widely used for T.sub.2 O storage is Zeolite 4A, having a 4A.degree. micropore aperture and a formula Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.4.5H2O. Zeolite 4A has the advantage of a high water storage capacity (about 25% by weight) and low vapour pressure of absorbed water. Zeolite 4A has a major disadvantage that water absorbed in the Zeolite rapidly exchanges with environmental water. In the event of a breach of containment, thereby exposing T.sub.2 O loaded Zeolite 4A to water, relatively rapid loss to the environment would occur. Typically T1/2 for exchange of water absorbed in Zeolite 4A with environmental water is 2-3 hours.
Zeolites having a longer T1/2 would clearly be useful in reducing the possibility of release of T.sub.2 O, or even of avoiding the need for a sophisticated container if the Zeolite could be dumped (in conformity with environmental legislation), for example in the sea. As the radioactive decay half-life of tritium is 12.3 years, a T1/2 measured in years would be highly desirable.
Among other known but little studied (as regards T.sub.2 O/H.sub.2 O exchange) Zeolites is Analcime. The formula of Analcime is normally expressed as Na.sub.2 O.Al.sub.2 O.sub.3. 4SiO.sub.2.2H.sub.2 O, but quite large variations are possible without substantially changing the crystal structure, eg the SiO.sub.2 /Al.sub.2 O.sub.3 ratio can very between 3.6 to 5.6 (D. W. Breck, Zeolite Molecular Sieves, Wiley, 1974). Analcime is not yet commercially available but methods of synthesis have been known for a long time.
The structure of Analcime is very constricted; the basic SiO.sub.4 and AlO.sub.4 tetrehedra mutually link to form 4 or 6 membered rings. The framework encompasses 16 cavities which form continuous non-intersecting channels with a pore aperture diameter of around 2.4A.degree., ie comparable with the diameter of a water molecule (ca. 2.6A.degree.). Analcime is known to retain about 9% by weight of water.
A fundamental problem with Analcime however is that no previously known synthesis method has been able to consistently produce Analcime in a crystal size larger than about 50 microns and most of the more convenient methods result in Analcime of a crystal size in the 10-20 micron range. Experiments on Analcime at a crystal size of 13 microns indicate a T1/2 for water exchange of about 2 years. Although this is clearly an advance on Zeolite 4A it still leaves room for improvement.
A method for producing larger crystals of Zeolites of formula A, (Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.XH.sub.2 O) and formula X (Na.sub.2 O.Al.sub.2 O.sub.3.2.8SiO.sub.2.XH.sub.2 O) is described by J. F. Charnell, J Cryst. Growth. (1971) 8 291-294. This method uses a low temperature (75.degree.-85.degree. C.) and a significant amount of time (2-5 weeks) and produces crystals of X having a maximum crystal size about 140 microns, but for A only about 60 microns.
It is an object of this invention to provide Analcime in a larger crystal size than has been hitherto available with the aim of providing increased half-life of water exchange. Other objects and advantages of this invention will be apparent from the account below.